For preparation of nitric acid, ammonia is generally first oxidized with air over a catalyst and nitrogen oxide NO is produced, which is oxidized later in the process to nitrogen dioxide NO2 [Behr, A., 2002. Ullmann's Encyclopedia of Industrial Chemistry, Vol. 6, Wiley-VCH, Weinheim].
Subsequently, the nitrogen dioxide NO2 thus obtained is absorbed in water to form nitric acid. In order that a maximum amount of the nitrogen dioxide NO2 obtained is absorbed by water, the absorption is generally effected at elevated pressure, preferably at pressures between 4 and 14 bar.
The oxygen required for the conversion of the ammonia used as the raw material is generally supplied in the form of atmospheric oxygen. For the purpose of supply, the process air is compressed in a compressor and brought to a pressure appropriate both for the oxidation reaction and for the absorption reaction.
Modern nitric acid plants are operated under pressure in order to achieve higher acid concentrations and higher absorption rates of NOx (i.e. better efficiencies) in the absorption. A distinction is made between two-pressure and one-pressure plants. In the one-pressure process, both the combustion and the absorption are performed at moderate pressure (<8 bar) or high pressure (>8 bar).
In the two-pressure plants, the gas production, i.e. the production of the nitrous gases by oxidation of ammonia, is effected under a pressure of about 4 to 6 bar, and the absorption of the nitrous gases thus obtained with water to give nitric acid typically at 8 to 12 bar.
The pressure is generated using compressors which are driven by means of a gas and/or steam turbine or electric motor. A gas turbine is preferably operated with the offgas of the nitric acid plant using the pressure applied by the at least one compressor.
Modern nitric acid plants are equipped with residual gas cleaners in order to satisfy the ever stricter regulations regarding the NOx and N2O offgas emission. Offgas cleaners are now state of the art because the nitrogen oxides NOx are responsible, for example, for “acid rain” and the offgas constituent N2O (laughing gas) is one of the greenhouse gases. With the systems available on the market for offgas cleaning, it is possible to reliably comply with the current limits for NOx emissions in nitric acid production in steady-state operation of the plant.
Examples of residual gas cleaning for the steady-state operation of nitric acid plants are known from an article by van den Brink entitled “Combined catalytic removal of NOx and N2O in a single reactor from the tail gas of a nitric acid plant” from Report No. ECN-C-02-009 (February 2002) and from the ThyssenKrupp Uhde company document “Setting Emissions Standards for Nitric Acid Plants”. None of these documents discloses measures which are taken in the course of startup and/or shutdown of a nitric acid plant in order to reduce the content of nitrogen oxides during this operation phase.
In contrast, in the course of non-steady-state startup and shutdown of the nitric acid plants, an elevated NOx concentration of, for example, >50 ppm in the residual gas currently arises in the outlet of the chimney unless additional measures for emission reduction are installed in the plant. The elevated NOx emissions become visible by a yellowish to deep brown color in the residual gas, according to the NO concentration at the outlet of the chimney. The higher the concentration, the darker and more intense the appearance of the color of the emerging gases. The elevated NOx concentrations in the residual gas in the exit of the chimney arise since the residual gas cleaning is normally not, or no longer, in operation when the nitric acid plant is started or when the nitric acid plant is not in operation. In addition, an elevated NOx concentration arises in the course of startup of the plant, since NO gases are still present in the plant, for example in pipelines, pipeline internals or other plant parts.
In contrast to steady-state operation, it is generally impossible at present in the course of the startup/shutdown operation of the nitric acid plants to avoid nitrogen oxide emissions which distinctly exceed the standard limits for a limited time. This involves emission of NOx predominantly as NO2, which is visually perceptible to an increasing degree as a brown offgas above the chimney from about 20 ppm. These operating states have to date been the subject of relatively little consideration because they are comparatively rare compared to the steady-state operating mode and public interest was comparatively low. Due to the increasing environmental awareness of the public and the resulting emission laws, plant operators are now also increasingly demanding “reduced emissions” or “colorless” startup/shutdown.
In the course of startup from the switched-off/cold state, the nitric acid plant is first filled with air (“air operation”) with the import of outside energy (for example outside steam or power). In the course of this, the offgas is heated to a temperature of about 165 to 200° C. at the inlet of the residual gas cleaning. The first NOx emissions arise as soon as the absorption tower, during the startup process, is filled with nitric acid from a reservoir vessel and the NO2 gas present in the acid is stripped out/blown out by the air. Current residual gas cleaning operations in which ammonia is used as the reducing agent for the NOx cannot be put into permanent operation until a minimum temperature exceeding 200° C. for the purpose of avoiding the formation of ammonium nitrate on the catalyst and in the downstream plant system. Moreover, the degree of NOx oxidation present in the course of startup is unfavorable for the residual gas cleaning operation. Therefore, in current plants, the NOx gas formed during the filling operation is emitted. With the end of the filling operation, NOx emission also ceases at first until the NH3 oxidation of the nitric acid plant is started (“ignited”). After the ignition, the temperature and NOx concentration in the plant rise constantly to the steady-state operating value, and the residual gas cleaning can be operated as planned from a temperature of about 200° C. From this operation point, the legally stipulated NOx emissions can be complied with.
Particularly the starting of the compressor set in the two-pressure process, high-pressure process and moderate pressure process and the starting of the ventilator in the atmospheric process have to be viewed critically, since the starting of the compressor set causes the first NOx emissions through the gas present in the plant at the exit of the chimney. This is the case particularly when the shutdown of the plant has been unplanned. The NOx emissions are caused firstly by NOx gases present in the plant system, and by outgassing NOx from the unbleached nitric acid in the absorption tower. NOx emissions resulting from outgassing NOx from the nitric acid are caused to a particularly high degree in the course of filling of the absorber with nitric acid shortly before the plant is started, since the acid is bleached by the air which is conveyed through the plant before the startup. Furthermore, considerable NOx concentrations are attained in the residual gas in the exit of the chimney after the plant has been started (ignition of the ammonia burner).
In the course of non-operation or in the course of shutdown of a nitric acid plant, the nitrogen oxides present under pressure in the plant, i.e. principally NOx (predominantly NOx and NO2) and N2O, are decompressed via the absorption column and the residual gas cleaning into the surrounding atmosphere. Since the residual gas cleaning can be kept in operation only for as long as permitted by the permissible limiting temperatures and hydrodynamic conditions, this cleaning is generally out of operation before the plant is completely decompressed. Furthermore, the absorption column, which is typically equipped with sieve trays, begins to become unstable with reduced gas flow, and so the absorption efficiency declines significantly. Experience has shown that, as soon as the residual gas cleaning is out of operation, the content of nitrogen oxides in the gas to be released to the atmosphere will increase significantly during the residual decompression, which will lead to the offgas becoming visible at the outlet of the chimney.
Therefore, solutions have already been sought, in which the nitrogen oxide offgas concentration in the course of shutdown and/or startup of a nitric acid plant can be reduced without the use of the conventional residual gas cleaning operation.
For reduction of the NOx offgas concentration in the course of shutdown and/or startup of a nitric acid plant, WO 03/078314 A1 proposes, in the course of shutdown of a nitric acid plant operated under pressure, after the residual gas cleaning has stopped, maintaining the pressure existing within the plant temporarily and then decompressing the gas in a regulated manner and releasing it to the environment diluted by means of air fed in from the outside. These measures can achieve regulated release of gases to the environment, these comprising NOx in sufficient dilution, such that it is possible to refer to colorless shutdown of the plant. In the course of restarting the plant, it is suggested that an elevated proportion of process air be supplied to the plant upstream of or beyond the residual gas turbine, such that the gas escaping from the plant has sufficient dilution of nitrous gases. A reduction in the content of nitrous gases emitted from the plant, however, is not achieved thereby. Moreover, the application of the process described in this document necessitates specific precautions within a nitric acid plant which would not be absolutely necessary for the “normal” operation of the plant. For example, it is necessary to install within the plant apparatuses which enable blocking-in, i.e. enclosure, of the gas present in the plant within the stoppage operation.
In the above-described process variant, the NOx-laden offgas is diluted with air to such an extent that it is no longer visible at the exit of the chimney. In spite of this, the absolute amount of NOx emitted to the environment per startup operation is maintained. This procedure is increasingly being perceived as no longer acceptable by the operators of the plants and by the legislators. The result is an urgent desire also to be able to prevent the NOx emissions even in the startup and shutdown operations.
In Dutch Notes on BAT for the Production of Nitric Acid, Final Report, December 1999, measures are described to reduce the emission of nitrogen oxides in the course of operation of nitric acid plants. As well as measures for steady-state operation, measures in the course of startup and shutdown of such plants are also described. These involve increasing the absorption capacity of the absorption tower, and the injection of steam into the chimney. The aim of both measures is to absorb the NOx present to form nitric acid and to avoid the emission thereof into the environment. Further measures mentioned are the heating of the offgas during the startup or shutdown, or the installation of an SCR plant (=plant for selective catalytic reduction) active at low temperatures. The aim of both measures is to be able to operate the steady-state residual gas cleaning operation as early as possible or for as long as possible. Further measures mentioned are the installation of a trickle tower or the installation of a tower with dry absorbent. The aim of both measures is to absorb the nitrogen oxides present and thus to prevent the emission thereof into the environment. Disadvantages are the high capital costs associated with the abovementioned measures.